Good news, bad news

[John Rose]

On May 23, 2011, at 3:44 PM, Tom Rodriguez wrote:

>> I'd *love* for intermediate static Java snippits like this to inline
>> straight through, even if invokeExact would be megamorphic under
>> normal circumstances...but I don't think that's the case right now,
>> right?
> 
> I haven't been following 292 that closely but it used to be a piece of code that did precisely that.
> 
> +        private Object invoke_L0() throws Throwable {
> +            if ((boolean) test.invokeExact())
> +                return target.invokeExact();
> +            return fallback.invokeExact();
> +        }
> 
> It looks like it was reworked at some point to use selectAlternative but the optimizer was never updated to deal with it properly.  It's not particularly hard, we just need to generate code like we would for a bimorphic call site.  The current code expects to either get a constant or something else and doesn't inline if it's something else.  In this case we have a Phi of two constants which we just need to split.

Yes, it would be a quasi-bimorphic call site keyed from a phi of two constants, instead of a profile of two receiver classes.

> I'm still unclear why you couldn't write your own variant of guardWithTest and have it work but my knowledge of what's really allowed is somewhat limited.

Those snippets will inline (I think), but at some point Charlie will want to fetch a bit of constant stuff out of an instance variable.  That will look non-constant to the optimizer, even if the instance variable is final and the enclosing object is a constant reference.  We made sure this happens for java.lang.invoke classes, but we haven't extended it yet to user code, in part because it would have its own bug tail to work through.

-- John